1. Field of the Invention
The present invention relates to devices for isolating one part of an electrical circuit from another. More particularly, the present invention relates to optocouplers for converting electrical signals into corresponding light signals and then back into electrical signals, thereby providing electrical isolation between two parts of an electrical circuit.
2. Discussion of Background
Electro-optical conversion devices are known and used for many purposes, including photoelectric conversion, light detection, optical data processing, and electrical isolation.
For example, with respect to photoelectric conversion, Hayashi et al, in U.S. Pat. No. 5,047,090, disclose a semiconductor device having a plurality of photoelectric conversion elements made of amorphous silicon for generating electric power.
Misumi et al, in U.S. Pat. No. 4,565,924, teach electro-optical conversion in an optical feedback, light signal binary device, that can be used as a light level detector or light relay in optical fiber transmission. The device converts input light signals to electrical signals, using a phototransistor or photodiode, and then converts the electrical signals back to optical output signals, using a ordinary LED.
Optical data processing systems, such as disclosed by Ulijasz in U.S. Pat. No. 4,549,085, can also use electro-optical conversion. In his data processing system, Ulijasz uses LEDs and photodiodes in emitter-detector pairs that are capable of processing both short and long wavelength optical signals.
In U.S. Pat. No. 4,539,480, Artinano et al describe an electrical isolation device for interrelated electrical circuitry. The device uses a pair of optically interconnected emitter-receiver systems disposed in input and output loop circuits. Also, the system output is related to the input current.
Optocouplers, often referred to as optical isolators, are well known electro-optical devices for translating or converting electrical signals into light signals, and subsequently back into electrical signals. Optocouplers are used to permit translation of data between electronic systems that may exist at differing ground potentials, at separated locations, or both.
An optocoupler essentially consists of three elements: a light emitter, a transmission medium, and a light detector. The emitter may be any device capable of converting electrical energy into light at virtually any wavelength or band of wavelengths. Similarly, the detector may be any device capable of detecting and responding to the emitted wavelengths and converting them to electrical signals or modulating externally-supplied current in a corresponding manner. The transmission medium must be an electrically-insulating material that is transparent to light at the wavelengths used. The separation between emitter and detector can range from a few millimeters, such as in a packaged optical isolator, to several miles, such as in a fiberoptic arrangement.
There are two known types of detectors used in optocouplers, each having different electrical characteristics. The first type is a photoconductor, or photoconductive cell, employing the use of a photosensitive material, such as cadmium sulfide, cadmium selenide, or a combination thereof.
The second type of detector is photodiode, which is essentially a photosensitive semiconductor. The photodiode absorbs photons and produces an electric current proportional to the number of photons.
The differing electrical characteristics make these detecting devices suitable for different applications. Photodiodes have a faster response, typically within nanoseconds, than do photoconductors. Also, photodiodes can produce power for use by other devices, while photoconductors cannot. However, photoconductors are more rugged, easier and less expensive to manufacture, and have higher voltage and power ratings. Additionally, a photoconductor appears electrically as a pure resistance with conductivity linearly proportional to the incident light intensity, and thus can be extremely versatile in control and signal processing applications, particularly where analog signals are involved.
However, conventional optocouplers, using these detectors and conventional light emitters, have limited application due to problems inherent in the combination of these components, such as wavelength mismatch and thermal-noise effects.
There is a need for an improved optocoupler having improved conversion characteristics, in particular, truly linear signal conversion.